From WO 01/30478, a liquid filter is known which has a functional support insert. Said functional support is suitable to supply and discharge the fuel to be cleaned to the filter element and to receive several further elements such as, e.g., a check valve. The functional support is seated in the lower part of the filter housing and unpurified oil flows around it. The filter housing is closed by a base which has openings for the different ports of the functional support. Moreover, said functional support has a drainage channel which is necessary for changing the filter. Said functional support is designed for the use in oil filters and thus is not suited for the use in a fuel filter.
In U.S. Pat. No. 7,040,299, a fuel filter is shown in which in the lower part of the filter housing, a strainer with integrated fuel heater is inserted on the dirty side. The filter housing has a plurality of inlets and outlets and connection lines. Since the filter also separates the water contained in the fuel, a water drain with water level sensor is provided. A disadvantage of this construction is that the discharged water is not sufficiently cleaned. In order to clean it to an extent that it is safe to discharge the water into the environment, a further component, e.g., external component is necessary.
From EP 1 581 736 and from DE 10 2006 039 581, a fuel filter is known which has a further filter attached on the filter housing for cleaning the discharged water.